This invention relates to glass fiber making and particularly to the type of fiber making in which a glass optical fiber is drawn from a preform.
Optical fibers are customarily drawn by feeding a glass preform longitudinally into a heating chamber, wherein one end of the preform is heated in a heating zone causing the preform to soften and begin to flow by gravity, thereby forming a fiber. The fiber is then drawn from the preform.
One of the most important considerations in the drawing of optical fibers is the total optical loss of the drawn fiber. In long distance communication systems, for example, a low loss characteristic (for example less than about 0.1 dB/km) is essential. Absorption and scattering losses, both intrinsic and extrinsic, contribute to the total optical loss in a glass fiber. In respect to intrinsic loss, intrinsic scattering loss results from density fluctuations in the glass, and intrinsic absorption loss arises from the glass multi-phonon structural vibration.
Fluoride glasses have been found theoretically to have an ultra low intrinsic optical loss, about 0.001 dB/km at a wavelength of 3.6 .mu.m. Therefore, fluoride glass has much potential as a material for use in low loss optical fibers suitable for long range communication systems. However, fluoride glass is very unstable and is susceptible to moisture attack, thus forming OH and O groups upon heating the glass to the fiber drawing temperature. The hydroxyl groups are a source of unwanted extrinsic absorption loss at wavelengths of about 2.8 .mu.m, whereas oxygen causes absorption of light at a wavelength of approximately 7.3 .mu.m.
In addition to the loss due to contamination by moisture and oxygen, loss can also result when the glass is exposed to the drawing temperature for an extended period of time or to a non-uniform drawing temperature, conditions causing phase separation and microcrystallization. Such crystallization in the fluoride glass causes scattering loss.
Prior methods of making fluoride optical fibers have not been capable of producing fluoride glass fibers having low loss characteristics, primarily because of crystallization of the glass from extended heating, fluctuations in drawing temperature, and glass contamination by impurities such as moisture and oxygen.
Typically in prior techniques, the crucibles or preform from which the fiber is drawn are flushed with an inert gas such as argon, as shown in FIG. 5 of "Fluorozirconate Glasses with Improved Viscosity Behavior for Fiber Drawing" by D. C. Tran, Ginther, and Sigel, Materials Research Bulletin, Vol. 17, No. 9 (1982), incorporated herein by reference. This inert gas serves only to purge the heating chamber of impurities such as moisture, oxygen, dust, transition metals, etc. Any contaminants which reach the surface of the glass remain in the glass, thereby causing undesirable optical loss in the finished fiber. In addition, prior drawing devices have heating zones, in which the preform is softened and drawn, which are too long, such that crystallization of the glass results. Finally, prior devices also do not provide adequate temperature stability for fluoride fiber drawing within the heating chamber, which also results in crystallization. As discussed, such crystallization in the finished fluoride fiber causes unwanted scattering loss.